Embodiments of the invention relate generally to a magnetic resonance (MR) coil elements and, more particularly, to decreasing interference among MR coil elements.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Often, a phased array is used during MR imaging. A phased array includes a plurality of radio-frequency (RF) coils or coil elements. Typically, the RF coil elements of an array are configured to minimize interference caused by cross-talk between other RF coil elements of the phased array. For example, interference may be caused by inductive coupling, where one coil element inductively induces a current into another coil element. Inductive coupling becomes more predominant as coil density or coil channel count increases. Generally, inductive coupling tends to increase correlated noise between coil elements of an array. As such, the signal-to-noise ratio (SNR) of each coil generally degrades, thus degrading the performance of the RF coil array. Cross-talk interference may also be caused by inductive coupling between transmit and receive coils. For example, inductive coupling caused by resonance between an MR whole-body transmit coil, which produces the excitation field, B1, and one or more receive coils can cause interference, thus degrading resulting image quality.
There are a variety of known techniques implemented to isolate or minimize cross-talking among coil elements of an array or among a transmit/receive coil combination of two or more coils. Traditionally, a circuit that includes an MR coil element, a feeding scheme that employs a matching network, and a low input Pre-amplifier (Pre-amp) are employed to reduce RF current in RF coils to improve the isolations between RF coils of an array, thus reducing interference. Blocking impedance resulting from such a circuit layout, however, is generally limited. For example, the actual blocking impedance, which is generated by the matching network, is often on the order of 100 to 500 ohms, depending on the coil size and loading. As coil element size decreases, so does the blocking impedance.
Another technique employed to minimize cross-talk between coil elements attempts to improve the blocking efficiency of matching networks by feeding RF coils in series with differential Pre-amps. However, a poor noise match between the differential Pre-amps and the RF coil may result in poor SNR. That is, poor SNR can be caused by a poor noise match between the high noise impedance of the differential Pre-amp and the impedance of the source.
It would therefore be desirable to have a method and system capable of at least providing an improved blocking impedance to RF coils in an RF coil array and having the ability to be noise matched with a variety of source impedances presented by RF coils without using a matching network.